We have identified a novel isozyme of lactate dehydrogenase, LDHk, which is expressed in anaerobically stressed rat fibroblasts and in rat and non-rat cells infected with the Kirsten sarcoma virus. This enzyme is uniquely regulated by oxygen and guanosine triphosphate. As isolated from ts KIMSV infected cells, it is thermolabile relative to that isolated from wild-type KiMSV infected cells. We intend to characterize this LDHk, both as isolated from rat cells and from KiMSV-infected non-rat cells. Basic kinetic parameters will be determined, and we will attempt to define the biochemical basis of the oxygen and GTP regulation. Apparent subunits will also be studied. Through use of the ts mutant, ts-371 KiMSV, we will confirm that this isozyme activity is at least in part encoded by the transforming gene fo the Kirsten sarcoma virus. We will also examine the enzyme from cells infected with revertant virus. In vivo studies will investigate expression of LDHk in normal rat tissues, in an attempt to define its normal physiological expression. Rat tumors will also be examined to determine if abnormal expression is associated with neoplasia.